Much like the engine concept itself, I've had many ideas going round and round in my head about the rotary displacer stirling engine concept.

First, I'm looking for validation in my thinking that the main issue rotary displacer stirling engines face is the lack of feasible regenerator. Without a reciprocating gas path, there isn't an easy way to deliver and retrieve heat from/to a regenerator.

Assuming I'm validated, has anyone considered a counterflow heat exchanger in two parts dividing the hot and cold parts of the engine. Imagine the round cross section of the engine divided into four parts with thermal breaks between them. Either a fluid flow, or a loop style heat pipe would move the heat from one side to the other, providing the function of the regenerator in the reciprocating design, but in a rotary style engine.

I agree about the problem of incorporating regeneration in rotary displacers. There's a similar related problem of the air moving from side to side without being forced through the heat exchangers. The disadvantages strike me as similar to thermal lag engines which have the same seductive simplicity. If you see a way to solve the problems without putting complexity back in, perhaps it could apply to both types of engines.

1) regenerator: A loop heat pipe using water as the heat pipe fluid (possibly some glycol to avoid freezing), moving heat from the 1/4 of the rotation after the hot side, and move it to the 1/4 of rotation after the cold side.

2) heat exchanger: using a cylindrical void in the rotating displacer that would induce a circular working fluid path. If needed, an inducer of some sort, like a small blower wheel being driven by the engine rotation, might reside in that void, to increase heat transfer with the walls of the 4 engine sections (hot side, cold side, regenerators for hot and cold)

3) Construction: I'm leaning towards stamped sheet metal, all laminated together either using through bolts or using a brazing technology. The displacer would also be the flywheel. Power piston would be attached to the side of the engine wall and moved by a crank on the shaft.

Imagine the cross section of the engine with the cylindrical displacer moving clockwise.

We would have 4 equal arc sections making up the full circle of the outside of the engine.

Starting at the top, we have the hot side heat exchange area, then the 'boiler' side of the regenerator, then cold, then the 'condenser' side and then back to the hot.

So if I'm correct, the working fluid in the engine would get heated, expand, then enter the 'boiler' regenerator area, where it would give up heat to the heat pipe. Already pre-cooled, it would then get cooled by the cold side and full contract. Then it would regain heat in the 'condenser' side of the regenerator, that it had given up in the 'boiler' side of the regenerator. It would be preheated, and then gain more heat in the hot side.

Is that a thermal short? In my mind it is moving heat from one side to the other, and moving 'cool' from that side back, acting just as a regenerator would. I'm not saying I'm right with certainty, but I think I might be, unless I'm envisioning this all wrong. Am I?

I see your point and agree it could have some regenerative affect. If the regenerator segments are thermally bridged but insulated from the heater and cooler segments it shouldn't be a direct short. Of course, there's always some thermal shorting in any displacer chamber.Since the title of the thread says crazy rotary Stirling ideas, here's one that came to me: No piston or diaphragm at all; just a port to a pulse chamber with a turbine on the displacer covering the port. The blades on half of the turbine would be oriented one way and then switch orientation to maintain the same rotation as the flow reverses.

Awesome! my first thought was to split the displacer into two 'half engines' and put the power pistons between the two, right there in the middle of the displacer. The power pistons would really be 'power rockers' and would have a ratcheting mechanisms that would transfer the pressure difference to rotational motion. But... this would have complex sealing issues (especially at the ends of the rotary displacer), so I'm trying to roll it back to testing only one or two new design concepts at a time.